The objective of this program is to understand the role of human orbital fibroblasts (OF) in the pathogenesis of thyroid-associated ophthalmopathy (TAO). TAO is a disfiguring and sight-threatening inflammatory manifestation of Graves' disease (GD). In GD, activating antibodies (called TSI) are directed against the TSH receptor (TSHR) and are thought to drive development of TAO. We have established that OF in TAO comprise a heterogeneous population. This results from bone marrow-derived CD34+ fibrocytes inhabiting the TAO orbit. Fibrocytes express high levels of TSHR and thyroglobulin (Tg), two ?thyroid specific? self-antigens. This expression is driven by the transcription factor, autoimmune regulator (AIRE). Fibrocytes activated by TSH or TSI produce high levels of IL-1? and IL-6. Fibrocytes are identified in the TAO orbit as CD34+ OF that interact with residential CD34- OF. In the presence of CD34- OF, CD34+ OF express substantially lower levels of AIRE, TSHR, and Tg. Further, cytokine inductions are dramatically reduced. When sorted into pure CD34+ OF and re-cultured, they again express relatively high levels of AIRE, TSHR, and Tg, and the TSH/TSI induction of cytokines resembles that in fibrocytes. We present evidence identifying a factor produced by CD34- OF that attenuates the inflammatory phenotype of CD34+ OF as the neuron guidance glycoprotein, Slit2. Further, rhSlit2 down-regulates AIRE, TSHR, and Tg expression and abrigates cytokine induction in fibrocytes. We also present evidence that cytokines produced by TSH/TSI-activated fibrocytes polarize and expand T cell development skewed toward the Th17 paradigm, including IL-23, IL-6, and IL-1?. Organizing hypothesis: If Slit2 from CD34- OF in the TAO orbit fails to adequately attenuate the inflammatory phenotype of CD34+ OF, severe TAO is manifested. In contrast, if Slit2 adequately down-regulates CD34+ OF, TAO does not develop or is mild. The rationale for the proposed studies is that identifying the factors modulating the inflammatory phenotype of fibrocytes and CD34+ OF will lead to specific and effective therapies. We now propose: Specific Aim 1: Test the hypothesis that the change in circulating fibrocyte phenotype to that of CD34+ OF is mediated by the Slit2/ROBO1 pathway by 1) determining whether Slit2 is the factor from CD34- OF that down-regulates AIRE, TSHR, and Tg expression and attenuates cytokine induction by TSH/TSI in CD34+ OF; 2) determining the mechanism of Slit2 action in fibrocytes and CD34+ OF; 3) determining mechanisms underlying Slit2 expression/regulation in CD34- OF and quantifying ROBO1 levels on fibrocytes; 4) testing other fibrocyte inhibitors. Specific Aim 2: Test the hypothesis that TSH/TSI-activated fibrocytes and CD34+ OF generate specific cytokines that polarize T cells toward the Th17 paradigm by 1) determining the mechanism though which TSH/TSI induces IL-23 in fibrocytes and CD34+ OF; 2) determining whether co-culture of autologous fibrocytes and T cells results in Th17 development and identifying fibrocyte-generated cytokines that promote Th17 development. These studies should reveal determinant mechanisms for abrogating the severity of TAO.